Fossil trackway reveals avian characteristics of theropod posture

ResearchBlogging.orgIt’s often claimed that intermediary fossils representing stages in the divergence of extant taxa are absent from the fossil record.  Such claims are often made by those with a creationist bent, and in some cases on the back of spectacular ignorance of biology.  For example Adnan Oktar’s (he publishes bizarre anti-evolution tirades under the pen name Harun Yahya) stupid offer of several trillion Turkish Lira (apparently  many times the GDP of is home nation, Turkey) to anyone who can demonstrate an intemediary fossil.  Oktar’s concept of an intermediate fossil is of course quite bonkers (Richard Dawkins has an excellent demolition of this concept) in which intermediate forms are, for example, half starfish, half fish.  In reality, of course, intermediate fossils are known, though I suppose each gap filling fossil must generate two new gaps in the fossil record!

The evolution of birds from theropod reptiles is well established, from the fossil record (this being one of the great cases of transitional fossils), with many fossils notable for the presence of feathers.  This paper analyses fossilised footprint tracks, and interprets them as being made by reptiles of this type, and makes some interesting inferences on resting posture as well as the gait of these animals.  Trace fossils attributed to theropods are quite rare, and usually don’t include impressions made by resting animals.  In this case, the authors conclude that resting postures (which reflect the anatomical arrangements of limb and other joints) of theropods indicate avian characteristics much earlier that previously recognised.The 200 million year old traces that form the subject of this paper are found in southwest Utah, and are part of a longer trace that mostly comprises a record of a moving animal with impressions of hind feet with occasional tail drag impressions.  In the figure below, one of the studied strata is shown – you can see the footprints represented as little “crows feet”.  The set of impressions studied in the publications is indicated in red.  I think the tail impressions are the red lines, while the resting impression is to the bottom of the figure.  I have to confess that the technical terminology used in the paper can be a little opaque to non-specialists (such as me)!

Schematic map of the “Top Surface” tracksite (SGDS.18).  Beige shaded areas represent the “Top Surface” of the Main Track-bearing Sandstone Bed; gold shaded areas are unexcavated; brown areas represent areas removed after mapping to examine lower horizons. The Eubrontes trackway that includes the crouching trace is highlighted in red.

Schematic map of the “Top Surface” tracksite (SGDS.18). Beige shaded areas represent the “Top Surface” of the Main Track-bearing Sandstone Bed; gold shaded areas are unexcavated; brown areas represent areas removed after mapping to examine lower horizons. The Eubrontes trackway that includes the crouching trace is highlighted in red.

The authors describe the significant resting trace fossil as resulting from an animal which, when moving up a 10 degree slope

[...] then stopped, placing both feet parallel. It then lowered its body, bringing the metatarsals and ischial callosity into contact with the substrate, creating nearly symmetrical, elongate “heel” and circular ischial impressions.

Unusually, and because the animal was resting on a slope, there are impressions made by the fore-limbs.  These are uniqe, with medially-directed digits.  This argues for a joint arrangement in these animals inconsistent with reconstructions in which the fore-limb palms are oriented ventrally, and in favour of one in which the palms are medially oriented.  You can see this arrangement in the figure below, showing a reconstruction.

Restoration of Early Jurassic environment preserved at the SGDS, with the theropod Dilophosaurus wetherilli in bird-like resting pose

The authors conclude that these movement traces represent evidence of remarkably avian-like joint arrangements and posture in theropods, much earlier than previously thought.  It’s possible that the arrangement reflects a function in grabbing and holding prey.  In any case, in among the rather technical and anatomical descriptions in the paper, the description of the animal’s movements as it made the impressions in the substrate are really quite appealing.

Milner, A., Harris, J., Lockley, M., Kirkland, J., & Matthews, N. (2009). Bird-Like Anatomy, Posture, and Behavior Revealed by an Early Jurassic Theropod Dinosaur Resting Trace PLoS ONE, 4 (3) DOI: 10.1371/journal.pone.0004591

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